slotmove: fix handling for EAPI 4-slot-abi

This is just a really minimal fix, in order to prevent slotmove from
behaving incorrectly with packages that use EAPI 4-slot-abi. Any
slotmove commands that try so specify a sub-slot are treated as invalid
for now, since that will required additional EAPI conditional logic, as
reported in bug #426476.
7 files changed